1. Field of Invention
The present invention relates to a bearing assembly utilizing a fluid dynamic pressure provided in a motor.
2. Description of the Related Art
A recording disk drive such as a hard disk drive includes a spindle motor (hereinafter referred to as “motor”) for rotating a recording disk. The motor uses a bearing assembly utilizing a fluid dynamic pressure. In many cases in the bearing assembly, a thrust dynamic pressure bearing mechanism and a radial dynamic pressure bearing mechanism are provided between a shaft or a component of the bearing assembly connected to the shaft and a sleeve into which the shaft is inserted.
The structure of an exemplary known bearing assembly includes an inner sleeve and an outer sleeve combined to form a sleeve, and a shaft is inserted into the sleeve. A radial bearing is formed between an outer peripheral surface of the shaft and an inner peripheral surface of the inner sleeve. Thrust bearings are provided at an upper end surface and a lower end surface of the inner sleeve, respectively. A vertically extending path for fluid is provided between the inner sleeve and the outer sleeve. Annular taper seals are formed between the outer sleeve and a sealing plate opposed to the upper end surface of the inner sleeve, and between the outer sleeve and a flange portion of the shaft opposed to the lower end surface of the inner sleeve.
There is another known bearing assembly. In this bearing assembly, a space is defined by the sleeve fixed to a base and a counter plate which closes a bottom opening of the sleeve. The shaft which is to rotate, and a thrust plate which is opposed to the counter plate with lubrication fluid interposed therebetween are accommodated in the space. A fluid circulating path is formed between the shaft and the thrust plate and brings a pair of thrust dynamic pressure bearing mechanisms formed on and below the thrust plate into communication with each other. In still another exemplary bearing assembly, a pair of thrust dynamic pressure bearing mechanisms are formed between a cylindrical sleeve having a bottom and a lower surface of an annular thrust portion which is formed integrally with the shaft, and between an upper surface of the thrust portion and an annular upper plate which closes an opening of the sleeve. A through hole is formed in the thrust dynamic pressure bearing mechanism from its inner peripheral edge to its intermediate portion so as to extend in an axial direction parallel to a center axis of the bearing assembly.
In a dynamic pressure groove in an outer peripheral portion of the thrust dynamic pressure bearing mechanism, a negative pressure is locally generated due to a difference in a flow velocity of lubrication oil or the like, and air bubbles generated by the negative pressure build up in some cases. The air bubbles cause troubles such as NRRO (Non Repeatable Run Out) due to abnormal vibration when the motor rotates, thus adversely affecting reading and writing of information from and on a recording disk and a lifetime of the bearing assembly.
In the aforementioned known bearing assemblies, however, the thrust dynamic pressure groove and the circulation path for the lubrication oil are formed in the rotating component, and the position of the circulation path is fixed with respect to the thrust dynamic pressure groove. Therefore, generation of the negative pressure can be minimized in a certain dynamic pressure groove, but it is not possible to supply the lubrication oil to all portions of the dynamic pressure grooves at which the negative pressure is generated.